J. ENTOMOL. SOC. BRIT. COLUMBIA 116, DECEMBER 2019 !40 Assessments of Rhagoletis pomonella (Diptera: Tephritidae) infestation of temperate, tropical, and subtropical fruit in the field and laboratory in Washington State, U.S. W. L. Y E E1 AND R. B. G O U G H N O U R2 ABSTRACT To understand the likelihood of any risk of apple maggot, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), to domestic and foreign fruit export markets, knowledge of its host plant use is needed. Here, assessments of R. pomonella infestation of temperate, tropical, and subtropical fruit were made in the field and laboratory in Washington State, U.S. In field surveys in 2010– 2017 in central Washington, 6.7% of Crataegus douglasii and 6.1% of feral Malus domestica trees (both temperate plants) in fly-managed (insecticide- treated) sites were infested by larvae. In unmanaged sites, 54.1% of C. douglasii and 16.3% of feral M. domestica tree samples were infested. In field surveys of 36 types of temperate fruit in 2015–2018 in southwestern Washington, new host records for R. pomonella were one species and three hybrids of Crataegus, as well as Prunus domestica subsp. syriaca – all of which produced adult flies. In addition, Prunus avium was a new host record for Washington State, producing one adult fly. Prunus armeniaca x Prunus salicina and Vitis vinifera exposed to flies in the laboratory produced adult flies. Of 37 types of tropical and subtropical fruit hung in fly-infested M. domestica trees in southwestern Washington, only Mangifera indica produced puparia. Out of nine tropical and subtropical fruit types in laboratory tests, Musa acuminata x balbisiana produced puparia but no adult flies. Results provide a basis for further research and hypotheses concerning host use by R. pomonella and its potential impact on protecting both U.S. and tropical and subtropical fruit markets. INTRODUCTION The apple maggot, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), is a quarantine pest of cultivated apple (Malus domestica) (Rosaceae) in western North America whose ancestral hosts are hawthorns, Crataegus spp. (Rosaceae) (Bush 1966). Native to eastern North America and Mexico, R. pomonella in western North America was first detected in M. domestica in 1979 in Portland, Oregon, in the Pacific Northwest (PNW) of the U.S. (AliNiazee and Penrose 1981). It is now found throughout the PNW west of the Cascade Mountain ! 1Corresponding author: United States Department of Agriculture-Agricultural Research Service, Temperate Tree Fruit & Vegetable Research Unit, 5230 Konnowac Pass Road, Wapato, WA, 98951; [email protected] 2Washington State University Clark County Extension, 1919 NE 78th Street, Vancouver, WA, 9866 J. ENTOMOL. SOC. BRIT. COLUMBIA 116, DECEMBER 2019 !41 Range, a relatively humid region with little commercial tree fruit production. However, arid or semi-arid regions in the PNW east of the range in central Washington State (Washington), Oregon, and Idaho, as well as in British Columbia, Canada (Canadian Food Inspection Agency 2016), where commercial apples are grown are mostly free of the fly. Preventing fly movement from infested to pest-free areas across the PNW and preventing increases in fly numbers within quarantine areas are high priorities for U.S. state departments of agriculture. To date, there have been no reports of commercially grown apples from the PNW infested by R. pomonella larvae (Washington State Department of Agriculture 2018). Washington is the PNW’s biggest apple producer. It exports about 30% of its crop, which is valued at ≈US$2.26 billion a year (NASS 2017), to overseas markets (Anonymous 2018). About 20 of 60 export markets have requirements or restrictions for apple import due to R. pomonella. These markets include China, Japan, South Korea, India, South Africa, Indonesia, Australia, Brazil, Chile, and Mexico. Some of these countries include regions with humid, subtropical climates. Although R. pomonella is a temperate species, ecological niche models indicate there are marginal to favorable habitats for the fly in such climates (between 15°N and 30°N latitude; e.g., southern China, northern Laos, Vietnam, and the Philippines) (Kumar et al. 2016). Furthermore, low temperatures are not a requirement for adult emergence (AliNiazee 1988). Thus, R. pomonella might become established if it were introduced into a subtropical country. To understand the likelihood of any risk of R. pomonella to domestic and foreign fruit export markets, knowledge of its host-plant use is needed. Rhagoletis pomonella is known to develop in at least 60 plant taxa (Yee and Norrbom 2017), therefore movement of these taxa needs to be restricted. However, more knowledge of its host-plant use could help to further reduce the perceived risk. Areas in which greater knowledge is needed include (1) frequencies of infestations of Crataegus douglasii (black hawthorn) and M. domestica trees, (2) additional host plants infested by the fly, and (3) fly infestation of tropical and subtropical fruit. With respect to (1), C. douglasii and feral M. domestica occur spottily around commercial apple orchards in central Washington, where <10% and <1%, respectively, of trees were found to be infested by R. pomonella in 2004–2006 (Yee 2008). This suggests frequencies of infestations of both species are low, but that C. douglasii – which is native to the region – is more frequently infested and thus a greater source of flies. However, reassessments of infestation frequencies of the two plants over time may show that the frequencies change and thus can affect the trees’ importance in fly control. In addition, Washington State Department of Agriculture (WSDA) and county pest boards detect R. pomonella in C. douglasii and feral M. domestica trees at sites near apple orchards using traps and then treat fly-positive trees with insecticides, but these entities do not control flies in C. douglasii and feral M. domestica trees at sites farther from commercial orchards. Whether differences in frequencies and patterns of larval infestations of C. douglasii versus M. domestica trees in fly-managed (insecticide-treated) and unmanaged sites occur has yet to be determined. J. ENTOMOL. SOC. BRIT. COLUMBIA 116, DECEMBER 2019 !42 With respect to (2), there may be host plants of R. pomonella that are not yet identified that could increase the risk of the fly spreading should their fruit be moved from infested to uninfested areas within the PNW. Crataegus (hawthorn) species would be likely candidates, as 30 of the 60 plant taxa that are hosts for R. pomonella belong to this genus (Yee and Norrbom 2017). With respect to (3), R. pomonella attacks Mangifera indica (mango) (Anacardiaceae) and Carica papaya (papaya) (Caricaceae) hung in M. domestica trees in the field and in the laboratory and are suitable hosts that produced adult R. pomonella (Yee and Goughnour 2017). However, suitability of other tropical and subtropical fruit has not yet been determined. No commercial tropical and subtropical fruit belong to the Rosaceae, and because they differ from Crataegus and Malus species in many respects, they may not be attractive to R. pomonella. If that is the case, there may be no (zero) or minimal threat of R. pomonella attacking most tropical or subtropical fruit in subtropical environments. Here, our objective was to assess R. pomonella infestation of temperate, tropical, and subtropical fruit through surveys and tests in the field and laboratory in Washington. Specific goals were to determine (1) R. pomonella infestation frequencies of C. douglasii versus feral M. domestica in central Washington, both in fly-managed and unmanaged sites; (2) whether there are unrecorded host plants of the fly in southwestern Washington; and (3) whether various tropical and subtropical fruit are suitable as developmental hosts for R. pomonella. MATERIALS AND METHODS Field surveys. In July to September 2010–2017 in central Washington, field surveys were conducted of infestation by R. pomonella of C. douglasii versus M. domestica. Fruit from C. douglasii and feral M. domestica were collected at 10 sympatric sites (Appendix 1). Within each site, trees of the two species were ∼10–5,000 m apart, with numbers of each varying widely across sites. Sites were in sagebrush, bunchgrass, or ponderosa pine ecosystems (Lyons and Merilees 1995). The three fly-managed sites – in arid sagebrush and bunchgrass habitats – had an active fly detection and control program using insecticides run by WSDA and county pest boards. The seven unmanaged sites – mostly in less arid ponderosa pine habitat – had no history of fly control or had no control for up to 20 years before surveys. Each site was sampled for 1 to 3 years from 2010–2015. Exceptions were Klickitat, which was sampled only in 2010 and 2012, and Nile, where an additional C. douglasii sample took place in 2017. Both tree species occurred along creeks, along roadsides beside ditches, in meadows, along trails in wooded areas away from creeks, and in pastures. Fruit from both species were collected when ripe: C. douglasii from mid-July to late August, and M. domestica from mid-August to early October. About 800 C. douglasii fruit were picked per tree, depending on fruit load. About 50 M. domestica fruit were collected from beneath each tree about 1 week after they had dropped. In July to November 2015–2018 in southwestern Washington, field surveys were conducted of 36 types of temperate fruit – mostly non-native species – including hybrids, subspecies, and varieties (see Table 1 for a list of temperate fruit surveyed or tested for infestation by R. pomonella). The focus was on fruit of unrecorded hosts, but fruit of known hosts were also collected for comparison. J. ENTOMOL. SOC. BRIT. COLUMBIA 116, DECEMBER 2019 !43 Fruit types collected included most of the accessible fruit present at the 10 sites in the coast forest ecosystem (Appendix 1). Collections were made in parks, along roadsides, in urban areas, and in demonstration tree plantings.